A seismic refraction/wide-angle reflection survey for the Trans-Alaska Crustal Transect program reveals a thin, refractive crust beneath the southern Yukon-Tanana terrane (YTT) in east central Alaska. These data are the first detailed refraction survey of the southern YTT and compose a 130-km-long reversed profile along the Alaska and Richardson highways. Results from this study indicate that low-velocity (≤6.4 km/s) rocks extend to approximately 27 km in depth. Based on these low velocities and an average Poisson's ratio of 0.23 determined for depths of ≤27 km, an overall silicic composition is interpreted for this portion of the crust beneath the Yukon-Tanana terrane. From approximately 8 to 27 km depth the crust exhibits an increase in reflectivity. This middle to lower crustal reflectivity is modeled as alternating high- and low-velocity lamellae with an average velocity of 6.1 km/s at 10 km depth to an average velocity of 6.4 km/s at 27 km depth. Beneath these reflective, low-velocity rocks a 3- to 5-km-thick, 7.0 km/s basal crustal layer produces a prominent reflection that extends to offsets of up to 280 km. The crust-mantle boundary, modeled at an average depth of 30 km, produces a variable PmP reflection, which may indicate lateral heterogeneity of this boundary, and a weak and emergent Pn refraction with a velocity of 8.2 km/s.

We interpret the crustal section as follows: the low-velocity rocks of the southern YTT extend from the surface to depths of approximately 10 km; underthrust Mesozoic flysch of the Kahiltna terrane, rocks of the Gravina arc, and basement of the Wrangellia(?) terrane extend from 10 to 27 km depth; a 3- to 5-km-thick layer of mantle-derived mafic rocks, relic oceanic crust, or Wrangellia(?) terrane lower crust extends from 27 to approximately 30 km depth; a tectonically young Moho beneath the southern YTT is found at an average depth of 30 km; and it is underlain by a mantle that may be relatively cool and/or olivine rich. In this interpretation, the Yukon-Tanana terrane is a thin-skinned terrane. Our results indicate that tectonic, and possibly magmatic, underplating has played a significant role in crustal growth for central Alaska. ¿ American Geophysical Union 1992